Solid oxide fuel cell (“SOFC” below) operate at relatively high temperatures, using an oxide ion conducting solid electrolyte as an electrolyte, with electrodes placed on both sides thereof, and with fuel gas supplied to one side thereof and oxidizer (air, oxygen, or the like) supplied to the other side thereof.
In such SOFC, steam or CO2 is produced by the reaction between oxygen ions passed through the oxide ion conducting solid electrolyte and fuel, thereby generating electrical and thermal energy. The electrical energy is extracted from the SOFC, where it is used for various electrical purposes. At the same time, thermal energy is transferred to the fuel, SOFC, oxidant, etc., and used to raise the temperature thereof.
In conventional SOFC, when a microprocessor-based meter in a fuel gas supply system detects an anomaly during operation, or when anomalies due to earthquakes or the like arise, or when maintenance of auxiliary devices and the like are executed, it is necessary to temporarily halt operation. After such anomalies or other temporary factors have been resolved, or after him and maintenance is completed, prompt resumption of operation in as little time as possible is sought to ensure stable electrical generation.
In order to bring about prompt resumption of operation in fuel cell systems it has been proposed, for example in Patent Citation 1, that for conventional SOFC that when a restart of the fuel cell system is requested during a predetermined control process, the control system, rather than executing the first startup processing routine after executing all of the fuel cell system stop processing routines, should first transition to the point in time at which conditions are the same as for the point in time at which the call for restart was made, and then execute the restart process.
On the other hand, in the conventional SOFC set forth in Patent Document 2, it is proposed that thermal efficiency can be raised by housing the fuel cell stack in a housing container, while heating can be accomplished by heating with higher than conventional temperature fuel gases through combustion of excess gas in the housing container, thereby obtaining thermal quantities required for steam reforming when in a low load operation. To speed up operation in the conventional SOFC, a heating operation to heat the fuel reformer is performed when the fuel reformer temperature is less than the partial oxidation reaction starting temperature upon startup; when the temperature of the fuel reformer rises to a temperature band equal to or greater than the partial oxidation reaction starting temperature and less than the temperature at which steam reforming can occur, the fuel reformer is heated by reaction heat from partial oxidation and combustion heat from the fuel gas, thereby performing a partial oxidation reforming reaction (“PDX” below). Furthermore, when the temperature of the fuel cell rises to a temperature band at which steam reforming can occur, below the steady state temperature, reaction heat of the partial oxidation reaction, combustion heat from the fuel gas, and heat absorption by the steam reforming reaction are controlled to heat the fuel reformer, and an auto-thermal reforming reaction (“ATR” below) is performed in which partial oxidation reforming and steam reforming are used together, such that when the temperature of the fuel reformer reaches a steady state, the fuel reformer is heated by combustion heat from the fuel gas, and a steam reforming reaction (“SR” below) is performed. In other words, in the conventional SOFC of this type, startup was executed by reforming fuel in the sequence of PDX, ATR, and SR as the temperature of fuel reformer rose at start up, thereby enabling stable and prompt start up.